Sales of glass-ceramic hob plates have been expanding continuously for several years. This success can be explained in particular by the attractive appearance of such plates, and by the ease with which they can be cleaned.
Thus, large quantities of plane electric hob plates are made out of glass-ceramic. These plates are initially made out of a plate of glass that is a precursor for glass-ceramic, and then they are cerammed.
Such plane plates of glass-ceramic are also made for gas cookers or for combined hobs: in particular combined gas-and-electricity hobs. It is necessary to provide openings or holes through the thickness of that type of plate through which the atmospheric gas burners are passed. These openings are provided of a diameter that is sufficiently large relative to the diameter of the burners concerned to avoid force being used during assembly, and thus to avoid the risk of breaking the plate. Said diameter generally lies in the range 40 mm to 95 mm. Similarly, smaller openings may be provided in that type of plate, in particular for receiving control knobs.
These openings of large or small diameter are generally made through the glass-ceramic precursor plate, prior to implementing the ceramming cycle. The openings are generally made by milling or by being cut out by water jet.
The presence of such openings through which there pass metal parts (bottom portion of burner, support rod of control knob) gives rise in use to two types of problem:
Solutions to these problems have been proposed in the prior art, and in particular the use of various types of metal collar on the perimeters of openings provided through the plate (with the plate not being bent in this context: i.e. with the plate being kept plane). Thus, patent application EP-A-0 715 125 discloses assembling a gas burner in the corresponding opening of a glass-ceramic plate by interposing a metal collar in the form of an eyelet between them, with the collar fitting astride the edge of the opening and thus protecting it mechanically. In addition, sealing is provided by means of a ring-shaped graphite part positioned at the interface between the plate and the metal collar. Although such a system is satisfactory concerning mechanical strength and sealing, it is complex to assemble and cannot be disassembled by the user. In addition, the plate cannot be cleaned completely since the edge of the opening is covered. Finally, the metal collar is relatively expensive, in particular because it is made up of a plurality of parts made by stamping, and because it also makes use of a plurality of additional gaskets or spacers, thereby giving rise to non-negligible extra cost for the cooking apparatus as a whole.
Proposals have also made to bend upwards the perimeters of the openings. The slope thus established around the opening makes it possible to prevent liquids infiltrating beneath the plate, with the liquids flowing down the slope. Openings with perimeters bent in this way--said perimeter being in particular substantially in the form of a truncated cone--are disclosed in patent U.S. Pat. No. 5,549,100. The method implemented for performing that bending is not described in detail. It makes use of a vacuum during the cycle in which the glass is cerammed. In as yet unpublished patent application FR 97/06114, the Applicant has described glass-ceramic plates with at least one opening at the top of a local bending in the plate; at least a portion of the perimeter of said opening being shaped mechanically. The perimeter of the opening is bent during the ceramming cycle and the mechanical shaping of said perimeter advantageously takes place prior to said ceramming cycle.
Finally, patent application FR-A-2 726 350 discloses glass-ceramic plates including at least one fold having a radius of curvature of less than 5 cm. Said fold is made in the glass-ceramic precursor plate and said plate is subsequently cerammed in the folded state.
At this point, it may be observed that glass plates are commonly subjected to local bendings, in particular for the purpose of preparing windscreens and windows for motor vehicles. Such bending is obtained by the mechanical action of an appropriate tool on the plate while the plate is brought to a temperature that is sufficient for its viscosity to present a value that is compatible with the desired bending (in general, a viscosity of the order of 10.sup.7.6 poises is sought). Such a method cannot be transposed in the context of glass-ceramic plates insofar as heating the glass-ceramic precursor plate inevitably initiates and disturbs the ceramming cycle of said plate.
With reference to the problems explained above specific to plates having openings, in a first aspect, the present invention proposes a method of bending the perimeters of openings in a glass-ceramic precursor plate (local bending seeking to raise said perimeter), which method is of the type described in application FR-A-2 726 350. Surprisingly, the Applicant has found that the "folding" method of FR-A-2 726 350 can be adapted to bending the perimeter of an opening that describes a closed curve without sharp angles. This is really surprising insofar as the person skilled in the art would have expected to be faced with problems of breakage, of weakening of the glass plate constituting a precursor and/or of the resulting glass-ceramic plate, given the high levels of tension generated in the bent area.